Determining the mechanisms that allow neoplastic cells to undergo uncontrolled proliferation is crucial to understanding carcinogenesis. Several recent studies indicate that protein phosphatases, like protein kinases, play important and specific roles in cell-cycle regulation; thus alterations in the expression or regulation of these proteins may affect cell-cycle progression and could even allow cells to undergo unregulated proliferation. A role for protein phosphatases in the tumor promotion process has also emerged from studies with okadaic acid, microcystin-LR, and calyculin A, three highly specific and potent inhibitors of certain serine/threonine protein phosphatases (PPases) that have tumor promoting activity. We have cloned and characterized three human PPases, and one, designated PP5, is particularly attractive for further studies. PP5 is potently inhibited by okadaic acid and calyculin A, two potent tumor promoters in mouse skin. However, unlike the other known PPases, which are expressed at constant levels, PP5 expression is high during log phase growth and low or undetectable in quiescent or differentiated cell cultures. The expression of PP5 is induced by the addition of serum and/or 17beta-estradiol in some cell types, and the inhibition of PP5 expression enhances the ability of both the p53 tumor suppressor protein and dexamethasone, a synthetic glucocorticoid agonist, to induce the expression of the p21 cyclin- dependent kinase inhibitor protein in A549 cells. Together these studies suggest that PP5 plays an important role in the regulation G1/S- phase transition. This proposal is designed to test the hypothesis that protein phosphatases play an important role in cell cycle progression; thus, the interference of their activity may contribute to the aberrant proliferative behavior of neoplastic cells. These studies will focus on the following specific aims: Aim 1. Further characterize the roles of PP5 in the normal and aberrant control of cell cycle progression. Aim 2.Characterize the relationship of PP5 in glucocorticoid mediated inhibition of cell growth and "cross talk" between signaling networks induced by hormones and growth factors. Aim 3.Characterize the mechanisms regulating the expression of PP5. Through these studies we hope to further characterize the role of protein phosphatases in the regulation of cell cycle and gain insight into the how protein phosphatases may be involved in the aberrant proliferation of neoplastic cells.